Currently, silicon thin film solar cells often use plasma enhanced chemical vapor deposition (PECVD) to construct single-junction or multi junction photovoltaic PIN film layers. This type of radio-frequency (RF) capacitively-coupled parallel plate reactor is commonly used in the thin film solar cell industry. Industry-wide, the electrode with a supporting frame is usually called a clamping unit, a holder, or a fixture, and the plasma chemical vapor deposition apparatus with installed holder(s) inside the chamber is often called the “deposition box,” i.e., the reactor chamber.
The holders have been widely used for making large-area thin film deposition of various materials, such as amorphous silicon, amorphous silicon-germanium, silicon carbide, silicon nitride and silicon oxide, etc. The silicon thin film solar cell sector is an important branch of the solar industry, and the holder becomes one of the core equipment of the solar industry. Holders for 13.56 MHz RF are widely used in high-speed amorphous silicon thin film deposition and have high efficiency and low process cost. With the rising demand for silicon thin film technology, more attention has been given to microcrystalline and nanocrystalline silicon thin film materials.
However, in a microcrystalline environment, plasma generated by 13.56 MHz RF may have low plasma concentration, low deposition rate, long deposition period to reach targeted film thickness, and significant background pollution. Thus, the prepared thin film often has high impurity and poor optical properties, which seriously affects the quality and performance of the products. How to make high-speed deposition becomes key for crystalline silicon thin-film technology to successfully serve the industry.
Very high frequency (VHF) is referred to the legitimate frequency which is twice or more of 13.56 MHz. In the industry, the VHF mostly used is generally in the range of 27.12˜100 MHz. However, in the capacitive discharge model, standing wave effect and skin effect caused by VHF become very obvious, and these effects become stronger when the driving frequency increases. Professor M. A. Lieberman of University of California, Berkeley made a thorough investigation on these two effects. His research results show that the critical condition for VHF PECVD deposition of uniform thin films is that the free space wavelength of excitation frequency (λ0) is much larger than the capacitive discharge electrode chamber size factor (X), and the skin depth (δ) is much larger than the thickness tolerance factor (η0). For example, on 1 m2 of discharging area and with an excitation frequency of 60 MHz, λ0≈X and δ≈η. Therefore, under this excitation frequency, the skin effect and the standing wave effect become very obvious, leading to an uneven discharge on the electrode plate of 1 m2. Thus, how to achieve a large area of uniform discharge driven by VHF is one of the technical problems to be resolved for the crystalline silicon thin-film technology.
This also caused great interest in the industry. In 2003, U.S. Patent 2003/0150562A1 disclosed a method using a magnetic mirror in the capacitively-coupled discharge to improve the inhomogeneity caused by VHF. Chinese patents 200710150227.4, 200710150228.9, and 200710150229.3 disclosed three electrode designs of VHF, applying different feed-in forms of VHF signals to obtain uniform electric fields.
However, the following problems may still remain: 1) The electrodes in the VHF-PECVD chamber have complex design structures; 2) One reason for the continuous improvement is that the constant assembly/disassembly and cleaning of the reaction chamber and electrodes can cause abnormal deformation of the electrodes; 3) The multi-point feed-in structures disclosed in the existing patents may have a small contact surface, which requires symmetrical paths of individual feed-in points and there is no contact between the bonding conductors at the feed-in points and the cathode plate. More specifically, a shield of isolation may be needed between the bonding conductor and the cathode plate for effective discharge. These structural designs have relatively harsh actual requirements, have too many determination factors for uniform discharge, and cannot meet the actual production needs such as disassembly and cleaning.
Therefore, for the equipment used by the industry, a single point feed-in becomes the mainstream design. But due to the standing wave effect and the skin effect, current single-point feed-in structures cannot meet the requirement for increasing the high feed-in frequency. Thus, further development and improvement may be needed to make the existing deposition holders more practical to meet the current market demand and to reduce the cost. Meanwhile, it is also a trend to use CVD holder system capable of processing or depositing multiple glasses. Therefore, it is of great practical significance for the industry to apply an effective VHF feed-in model to meet the demand of mass production and to enter the industrial production stage.